Acidic organic extractant is the most widely used extractant in industry, and RE, Ni, Co, Fe, Al, Cu, Zn, Cr, V, etc. can be separated and purified by acidic solvent extraction. The most commonly used techniques include: saponification of extractant, such as P507(HEHEHP), P204(D2EHPA), C272(DTMPPA), P229(HDEHP), C301(bis(2,4,4-trimethylpentyl) dithiophosphinic acid), C302(bis(2,4,4-trimethylpentyl) monothiophosphinic acid), naphthenic acid, isomeric acid, for extractive separation of rare earth in hydrochloride acid medium([1] symposium of rare earth chemistry, Changchun Institute of Applied Chemistry, 1982, science press.[2] Xu Guangxian, Rare Earth (the second edition (volume 1), metallurgical industry press, 2002, P542-547); Separation of all rare earths in ionic rare earth ore with moderate yttrium and rich europium (CN87101822); process of solvent extractive separation of mixed rare earth by ammoniated P507(CN85102210); process of continuous saponification of organic phase (CN95117989.6); separation and purification of yttrium oxide in saponified naphthenic acid-hydrochloride system (Xu Guangxian, rare earth (the second edition, volume 1), metallurgical industry press, 2002, P582, 590). All above extractants used for separation and purification are acidic organic extractants, whose metal extraction ability (distribution ratio) has an reverse relationship with equilibrium acidity of aqueous phase. Generally, the extracted metal ion exchanges with the hydrogen ion of acidic organic extractant, and the latter transfers to aqueous phase, raises the pH value of aqueous phase and thus reduces the metal extraction ability (distribution ratio). Therefore, the extractant has to be saponified by ammonium solution or NaOH to remove the hydrogen ion in organic phase (see reaction equation 1), and then separates and purifies metal ion through exchange (see reaction equation 2). It can be seen that large amount of ammonium is consumed in the extractive separation process, which increases the cost, and moreover, large amount of ammonium-nitrogen waste water is produced and causes severe pollution to water resources. 0.6-1.0 ton of liquid ammonium is need for separation and purification of 1 ton of rare earth oxide. What's more, because the concentration of the ammonium-nitrogen waste water is low, the difficulty and cost of recovery is so high that enterprises are unable to accept it. How to get rid of the pollution of ammonium-nitrogen waste water to environment is a pressing difficult problem in current rare earth separation and purification industry.HA+NH4+=NH4A+H+  reaction equation 13NH4A+M3+=MA3+3NH4+  reaction equation 2
HA represents organic extractant and M3+ represents trivalent metal ion.
In recent years, in order to avoid the pollution by ammonium-nitrogen waste water, some enterprises use NaOH instead of liquid ammonium for saponification of organic phase, which produces no ammonium-nitrogen waste water but large amount of sodium chloride waste water with high salinity, and doubles the saponification cost.
The Chinese invention patent application 200710163930.9 discloses the pretreating method of an organic extractant and its application technique, namely, size mixing of rare earth carbonate with water, or Ca and Mg contained alkaline earth metal ore with rare earth solution, to obtain a pretreated slurry. At certain temperature the organic extractant is pretreated, the rare earth ion in pulp is extracted into organic phase, and the obtained rare earth ion loaded organic phase is used for non-saponification separation of rare earth.
The Chinese invention patent application 200710187954.8 discloses a pretreating method of an organic extractant, product and its application technique. The organic extractant is directly mixed with rare earth solution and alkaline earth metal compound powder containing Mg and/or Ca, or with slurry (produced with water), for preextraction. The rare earth ion in aqueous phase is extracted into organic phase, the new born hydrogen ion dissolves the alkaline earth metal compound powder, and the obtained rare earth ion loaded organic phase is used for non-saponification separation of rare earth.
Both of the invention patents above use Ca and Mg contained alkaline earth metal minerals, or Mg and/or Ca alkaline earth metal compound powder or slurry, namely, the pretreatment or preextraction process of organic phase by Mg and/or Ca oxide, hydroxide, carbonate powder or slurry. Because the Ca and Mg contained alkaline earth metal minerals and its oxide and hydroxide product contains many impurities, such as Si, Fe, Al, etc. (generally with 2˜4% Si, 0.5˜1% Fe, and 0.3˜0.5% Al), Fe, Al, etc. is easily extracted into organic phase and spoils the quality of the product; Si exists as oxide or silicate, so it stays stable and doesn't take part in the reaction. It still exists as solid which partially precipitates to the bottom of the extraction tank and is partially mixed in organic phase as the third phase. In addition, the solid-liquid reaction speed is low and incomplete, so it would also leave some solid substance which forms the third phase in the pretreatment or extraction process and thus hinders the extraction process. Adoption high grade alkaline earth metal minerals with less impurity will increase the production cost greatly and thus make the operation of the enterprises impossible.
Contents of Invention
One purpose of the invention is to provide a use of aqueous solution of Mg(HCO3)2 and/or Ca(HCO3)2 in extractive separation and purification of metals with acidic extractant; the second purpose is to provide a new process of directed extractive separation and purification of metal by organic extractant with low cost and without discharge of ammonium-nitrogen waste water; and the third is to provide a preparation method of Mg(HCO3)2 and/or Ca(HCO3)2 solution.
According to the characteristics of acidic organic extractants such as P507, P204, C272, P229, C301, C302, naphthenic acid, isomeric acid, etc., the invention studies the application of Mg(HCO3)2 and/or Ca(HCO3)2 in metal extractive separation and purification. Acidic organic extractant, aqueous solution of Mg(HCO3)2 or/and Ca(HCO3)2, and metal solution are added into extraction tank step by step or simultaneously for pretreatment. Metal ion is extracted into organic phase, and after clarification, a loaded organic phase containing metal ion that is hard to extract is obtained. The loaded organic phase is then used to exchange with metal ion that is easy to extract during the extractive separation and purification process of metal elements. After several stages of extractive separation and purification the metal ion that is hard to extract is separated from meal ion that is easy to extract. Basic reaction equations are as follows:Mg(HCO3)2+2HA=MgA2+2CO2+2H2O  reaction equation 33MgA2+2Ma3+=2MaA3+3Mg2+  reaction equation 4MaA3+Mb3+=MbA3+Ma3+  reaction equation 5
Ma represents metal ion that is difficult to extract, and Mb represents metal ion that is easy to extract.
Mg or/and Ca ion transfers into aqueous phase in the pretreatment process and basically do not enter the extractive separation and purification process with organic phase, which keeps the equilibrium acidity of aqueous phase stable in extraction process and concentration of alkaline earth metals in metal products low.
The aqueous solution of magnesium bicarbonate and/or calcium bicarbonate, used by the invention as acidity balancing agent, adjusts the balancing pH value of the metal extractive separation and purification process which uses an acidic organic extractant, improves the metal extraction capacity of organic phase, and increases the concentration of metal ion in the loaded organic phase, namely, acidic organic extractants such as P507, P204, C272, P229, C301, C302, naphthenic acid, isomeric acid, etc., aqueous solution of Mg(HCO3)2 or/and Ca(HCO3)2, and metal solution is mixed step by step or simultaneously for preextraction. Metal ions are extracted into organic phase quantitatively, and after clarification a loaded organic phase containing metal ions is obtained. The loaded organic phase are then used for extractive separation and purification process of many metal solutions, and after several stages of extractive separation and purification, single metal compound or an enriched substance of several metals is obtained.
In the invention, cheap minerals such as magnesite, limestone, calcite, dolomite are calcined and then mixed for digestion, or Mg(OH)2 or/and CaO is mixed for digestion, or Mg(OH)2 or/and Ca(OH)2 is directly mixed with water (raw materials above contain 2˜4% of Si, 0.5˜1% of Fe, and 0.3˜0.5% of Al), after which slurry is carbonized by CO2 gas and aqueous solution of Mg(HCO3)2 or/and Ca(HCO3)2 is obtained; or magnesium salt is used as raw materials for preparation of Mg(OH)2 which is mixed for digestion with water and carbonized to obtain Mg(HCO3)2 solution. The Mg(HCO3)2 solution is then filtered to remove the impurities such as Si, Fe, Al, etc. and get a pure solution of Mg(HCO3)2 or/and Ca(HCO3)2 with both of Fe and Al less than 5 ppm. The pure solution, acidic organic extractant, and metal solution are then mixed for preextraction. Compared with patents above, the invention has the following advantages: (1) no third phase is formed during preextraction or extractive separation and purification process, and it doesn't introduce impurities such as Fe, Al, Si, etc., which can ensure the quality of the product. (2) Mg(HCO3)2 or/and Ca(HCO3)2 is used to accurately control the equilibrium pH value of the metal extraction process, the recovery rate of rare earth is high, the liquid-liquid reaction is faster than liquid-solid reaction, the extraction of metal is more complete and flow speed of liquid slurry is much easier to be accurately controlled than that of solid slurry. (3) Raw minerals of Ca and Mg or low grade oxide and hydroxide are directly used as raw materials. In the carbonization process Mg and Ca are changed into liquid, while impurities such as Si, Fe, Al, etc. are left in slag and removed by filtration. Therefore, there is no high requirement to the quality of raw materials of Ca or Mg, and the cost of raw materials is reduced greatly. In addition, the Mg contained aqueous raffinate from the pretreatment of organic phase is transformed by alkaline for the preparation of Mg(OH)2 which is returned for the preparation of Mg(HCO3)2 by carbonization. The CO2 from the reaction between Mg(HCO3)2 or/and hydrogen ion in pretreatment process, from the calcination of metal carbonate and metal oxalate, and from boiler can be caught and used for preparation of Mg(HCO3)2 or/and Ca(HCO3)2 by carbonization. Therefore, the resources are effectively recycled, the pollution of CO2 gas and waste water is evaded and moreover, the production cost of metal is reduced greatly.
The specific technical solution of the present invention are as follows:
A use of aqueous solution of magnesium bicarbonate and/or calcium bicarbonate in the process of extractive separation and purification of metals is disclosed in the present invention.
A use of aqueous solution of magnesium bicarbonate and/or calcium bicarbonate in the process of extractive separation and purification of metals is to use the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate as an acidity balancing agent in the extractive separation and purification process which uses an acidic organic extractant.
The process of extractive separation and purification includes the following steps: (1) the acidic organic extractant, the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate and a metal solution with the metal ion to be separated and purified are added into extraction tank step by step or simultaneously, and then a preextraction is realized in single or multiple stage concurrent and/or countercurrent way, during which the metal ion in aqueous phase is extracted into the organic phase. Equilibrium pH value of the aqueous phase is 1-6, an organic phase loaded with metal ion as well as aqueous phase loaded with magnesium and/or calcium ion are obtained; (2) the obtained organic phase loaded with metal ion is used for extractive separation and purification of metal solution containing 2 or more species of metal ions which includes the ones contained in the organic phase. After multi-stage extraction, washing and stripping process, the difficulty-extracted metal ion enter into the raffinate solution, while the easily-extracted ones enter into the stripping solution, thus a raffinate solution, a lotion and a stripping solution products with different metal ions are obtained; alternatively, the obtained organic phase loaded with metal ions is extracted directly by acid or alkali solution to obtain purified metal solution or slurry. The solution is used to produce metal compounds product by concentration crystallization or precipitation, or to produce single metal compound product by further extraction, while the slurry is used to produce compound product by filtration.
The optimized process of extractive separation and purification is as follows:                (1) The acidic organic extractant and the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate is added into extraction tank simultaneously, and then a extraction is realized in single or multiple stage concurrent and/or countercurrent way, during which magnesium and/or calcium ion exchanges with hydrogen ion in the organic phase. Equilibrium pH value of the aqueous phase is 3˜5. After clarification and phase separation, a magnesium and/or calcium ion-containing organic phase and a waste water are obtained; then the magnesium and/or calcium ion contained organic phase is mixed with a metal solution containing the metal ion to be separated and purified to realize preextraction in single stage or multi stage concurrent and/or countercurrent way, during which the metal ion is extracted into the organic phase. After clarification and phase separation, metal ion loaded organic phase and aqueous phase loaded with magnesium and/or calcium ion. Equilibrium pH value of the aqueous phase is 2.5˜4.5.        (2) The obtained organic phase loaded with metal ion is used for extractive separation and purification of metal solution containing 2 or more species of metal ion which include the ones contained in organic phase. After multi-stage extraction, scrubbing and stripping process, the difficultly-extracted metal ion enter into the raffinate solution, while the easily-extracted ones enter into the stripping solution, then a raffinate solution, a lotion and stripping solution products with different metal ion are obtained.        
Alternatively, the obtained organic phase loaded with metal ions is stripped directly by acid or alkali solution to obtain purified metal solution or slurry. The solution is used to produce metal compound product after concentrating crystallization or precipitation, or to produce single metal compound product by further extraction, while the slurry is used to produce compound product through filtration.
The metal ions in the technique solutions are at least one selected from the group consisting of Lanthanum, Cerium, Praseodymium, Neodymium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium, Yttrium, Nickel, Cobalt, Iron, Manganese, Chromium, Aluminum, Vanadium, Copper and Zinc. The commonly used metal elements are at least one selected from the group consisting of Lanthanum, Cerium, Praseodymium, Neodymium, Samarium, Europium, Gadolinium, Terbium, Dysprosium, Holmium, Erbium, Thulium, Ytterbium, Lutetium and Yttrium.
The acidic extractant in the technique solutions comprises of single or mixed extractants selected from acidic phosphorous extractant and carboxylic acid extractant, wherein the extractant is diluted by organic solvent, and the concentration of extractant is 0.5˜2.0 mol·L−1. The stated acidic extractant is single or mixed system whose components are selected from the group consisting of P507, P204, P229, C272, C301, C302, fatty acid, naphthenic acid and heterogeneous acid.
The content of magnesium oxide and/or calcium oxide in said aqueous solution of magnesium bicarbonate and/or calcium bicarbonate is 5˜100 g/L, wherein the optimal content is 5˜30 g/L. The contents of iron and aluminum are less than 5 ppm, respectively, while the optimal result for both is less than 2 ppm.
Single stage or 2-20 stages concurrent and/or countercurrent extraction is used in said extraction of step (1), during which the volume flow ratio or phase ratio of the organic phase to aqueous phase (including metal solution, aqueous solution of magnesium bicarbonate and/or calcium bicarbonate, and other aqueous solution) is 0.2˜10:1 (O/A), with single stage mixing time: 3˜30 minutes and clarifying time: 5˜60 minutes. The temperature in the extraction tank is 20˜50° C., the total concentration of metal ions in the loaded organic phase is 0.05˜0.3 mol/L and equilibrium pH value of the raffinate aqueous phase containing magnesium or/and calcium ions is 2.5˜4.5. There are two ways to mix organic phase and aqueous phase: one is that organic phase and magnesium bicarbonate and/or calcium bicarbonate is mixed thoroughly at first and then mixed with metal ions solution, and the other is that organic phase, magnesium bicarbonate and/or calcium bicarbonate, and metal ions solution is mixed directly.
In the step (1) of the optimized process, 1 to 1.5 mol/L acidic extractant and an aqueous solution of magnesium bicarbonate and/or calcium bicarbonate saturated with carbon dioxide is mixed to realize extraction in single stage or 2-10 stages concurrent and/or countercurrent way, during which the volume ratio of the organic phase to aqueous phase (O/A) is 0.2˜10:1, with single-stage mixing time 3˜30 minutes and clarifying time 5˜30 minutes. An loaded organic phase with 0.15 to 0.3 mol/L magnesium and/or calcium ions and waste water whose pH value is 2.5˜4.5 is obtained. The waste water is returned for the carbonization process. Then single stage or 2˜10 stages concurrent and/or countercurrent way is used in the extraction reaction between the organic phase loaded with magnesium and/or calcium ions and 0.1˜2.0 mol/L metal solution, during which the volume ratio of the organic phase to aqueous phase (O/A) is 0.2˜10:1, with single stage mixing time 3˜30 minutes and clarifying time 5˜30 minutes. Metal ions are extracted into organic phase, and after clarification, a loaded organic phase containing 0.1˜0.2 mol/L metal ions and a raffinate aqueous phase containing magnesium or/and calcium ions with pH value 2.5˜4.5 is obtained. The 0.1˜2.0 mol/L metal solution is the raffinate in the process of extractive separation and purification of step (2). The temperature in the extraction tank is controlled at 20˜50° C.
The loaded organic phase of step (2) is used for extractive separation and purification of metal solution containing 2 or more species of metal ions, during which 10˜150 stages fractional extraction is used, while 3˜20 stages concurrent and/or countercurrent way is used for stripping. The volume ratio of the organic phase to aqueous phase (O/A) is 0.1˜10:1, with single stage mixing time 3˜20 minutes and clarifying time 5˜30 minutes. The temperature in the extraction tank is controlled at 20˜80° C.
Said metal solution is chloride solution, nitrate solution, sulphate solution or a mixture thereof, with metal concentration of 0.1˜1.8 mol/L.
The carbon dioxide which released from reaction between the acidic organic extractant and the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate of step (1) is collected and returned for the preparation of the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate.
One method of preparation of the aqueous solution of magnesium bicarbonate and/or calcium bicarbonate in the technical solution is to calcine, digest and carbonize magnesium or/and calcium minerals which are at least one selected from the group consisting of magnesite, dolomite, magnesium carbonate and other minerals.
The roasting process of magnesium or/and calcium minerals is that the minerals are roasted for 1˜5 hours at 700˜1000° C. The process of digestion is that water is added into magnesium oxide and/or calcium oxide obtained after roasting to digest at 50˜95° C., for 0.5˜5 hours, according to the weight ratio of the liquid to solid: 1˜5:1 (calculated by the weight of water and magnesium oxide and/or calcium oxide), and then water is added to form slurry based on the weight ratio of the liquid to solid: 10˜200:1 (calculated by weight of water and magnesium oxide and/or calcium oxide). The process of carbonization is that the slurry is carbonized by carbon dioxide gas after digestion, with reaction temperature controlled at 0˜50° C. and reaction time 0.1˜5 hours. Pure aqueous solution of magnesium bicarbonate and/or calcium bicarbonate is obtained by filtration.
The second method of preparation of the aqueous solution of magnesium bicarbonate in technical solution is to add water to magnesium oxide to digest at 50˜95° C. for 0.5˜5 hours, with the weight ratio of the liquid to solid: 1˜5:1 (calculated by the weight of water and magnesium oxide). Water is added to form slurry or magnesium hydroxide and water is mixed directly based on the weight ratio of the liquid to solid: 10˜200:1 (calculated by weight of water and magnesium oxide and/or calcium oxide). Then the slurry is carbonized by carbon dioxide gas, with reaction temperature controlled at 0˜50° C. and reaction time 0.1˜5 hour. Pure aqueous solution of magnesium bicarbonate is obtained by filtration.
The third method of preparation of the aqueous solution of magnesium bicarbonate in the technical solution is to use a magnesium salt as raw material for the preparation of solution of magnesium bicarbonate, and the specific steps are:
Preparation of magnesium hydroxide: the magnesium salt solution or solid magnesium salt is dissolved in water, then liquid or solid alkaline compound whose alkalinity is stronger than that of magnesium hydroxide is added, and after reaction, magnesium hydroxide slurry is obtained, or magnesium hydroxide filter cake is obtained by filtration.
Preparation of aqueous solution of magnesium bicarbonate: the magnesium hydroxide slurry or magnesium hydroxide filter cake obtained from step (1) is mixed with water and then carbonized by carbon dioxide gas to generate an aqueous solution of magnesium bicarbonate.
Said magnesium salt in step (1) is at least one selected from magnesium chloride and magnesium nitrate, and the concentration thereof is 10˜300 g/L (calculated by magnesium oxide).
Said magnesium salt solution in step (1) is at least one selected from the group consisting of raffinate aqueous phase containing magnesium chloride and magnesium nitrate obtained from the process of extractive separation and purification, brine and seawater, and the concentration thereof is 10˜200 g/L (calculated by magnesium oxide).
Said alkaline compound in step (1) is at least one selected from the group consisting of calcium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide obtained from calcium oxide by digestion, and a mixture of calcium hydroxide and magnesium hydroxide obtained from light burnt dolomite by digestion.
Said alkaline compound in step (1) is calcium hydroxide obtained from calcium oxide by digestion or a mixture of calcium hydroxide and magnesium hydroxide obtained from light burnt dolomite by digestion.
The amount of the alkaline compound added in step (1) is 1˜1.5 times of the theoretical stoichiometric amount. The reaction temperature is 15˜95° C., and the reaction time is 10˜180 mins.
During the process of mixing magnesium hydroxide slurry or magnesium hydroxide filter cake with water, the weight ratio of the liquid to solid is 10˜200:1 (calculated by weight of water and magnesium oxide). The reaction temperature is controlled at 0˜35° C. in the process of continuous carbonation by carbon dioxide gas, and a pure aqueous solution of magnesium bicarbonate is obtained by filtration.
The method of preparation of the aqueous solution of magnesium bicarbonate is that at least one selected from the group consisting of calcium carbonate, limestone, marble and dolomite is sprayed by water and carbonized by carbon dioxide gas, and then a pure aqueous solution of magnesium bicarbonate is obtained by filtration.